ADDED   day02-input.txt
Index: day02-input.txt
==================================================================
--- day02-input.txt
+++ day02-input.txt
@@ -0,0 +1,250 @@
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+mphcuisvrnczxwkbgmtpeoylfa

ADDED   day02.rkt
Index: day02.rkt
==================================================================
--- day02.rkt
+++ day02.rkt
@@ -0,0 +1,66 @@
+#lang racket
+
+(require rackunit)
+
+(define input (file->lines "day02-input.txt"))
+
+;; Returns hash table where keys are unique chars in str,
+;; values are # of occurrences of that char in str
+(define (check-id str)
+  (for/fold ([seen (hash)])
+            ([c (in-list (string->list str))])
+    (hash-set seen c (add1 (hash-ref seen c 0)))))
+
+;; Returns hash table where keys are numbers, values are counts of
+;; how many ids in id-list that have a character that repeats exactly
+;; [key] times
+(define (id-counts id-list)
+  (for/fold ([counts (hash)])
+            ([id (in-list id-list)])
+    (for/fold ([each-count counts])
+              ([c (in-list (remove-duplicates (hash-values (check-id id))))])
+      (hash-set each-count c (add1 (hash-ref each-count c 0))))))
+
+(define (day01-part1 id-list)
+  (define counts (id-counts id-list))
+  (* (hash-ref counts 2 0) (hash-ref counts 3 0)))
+
+(module+ test
+  (define test-ids '("abcdef"
+                     "bababc"
+                     "abbcde"
+                     "abcccd"
+                     "aabcdd"
+                     "abcdee"
+                     "ababab"))
+  (check-equal? (day01-part1 test-ids) 12)
+  (check-equal? (day01-part1 input) 7533)) ; Correct answer for part 1
+
+(define part2-example
+  '("abcde"
+    "fghij"
+    "klmno"
+    "pqrst"
+    "fguij"
+    "axcye"
+    "wvxyz"))
+
+;; Returns a list of characters at positions where id1 and id2 match
+(define (compare-box-ids id1 id2)
+  (define id1-chars (string->list id1))
+  (define id2-chars (string->list id2))
+  (filter identity (for/list ([i (in-range (length id1-chars))])
+                     (if (equal? (list-ref id1-chars i) (list-ref id2-chars i))
+                         (list-ref id1-chars i)
+                         #f))))
+
+(define (day02-part2 ids)
+  (define (is-match? comparison-chars)
+    (equal? (length comparison-chars) (sub1 (string-length (first ids)))))
+  (define matches
+    (filter is-match? (map (curry compare-box-ids (first ids)) (rest ids))))
+  (if (not (empty? matches)) (list->string (first matches)) (day02-part2 (rest ids))))
+  
+(module+ test
+  (check-equal? (day02-part2 part2-example) "fgij")
+  (check-equal? (day02-part2 input) "mphcuasvrnjzzkbgdtqeoylva")) ; Correct answer for part 2